In order to ascertain the effects of stretching on internal structure of smooth muscle cell membranes, the nerve connectives of the mollusk Aplysia were stretched twice and three times their normal length, fixed, and either freeze-fractured or embedded in plastic. The connectives contain numerous, isolated smooth muscle cells (SMC). The sarcolemma of the SMC, like that of vertebrate smooth muscle, is indented in the form of numerous pits or caveolae. Unlike most other species, the pits leave a natural marker in the form of "bumps" about 65-130 A wide inside their membranes. In the stretched connectives, the fractured sarcolemmas display many more clusters of these "bumps," indicating that the caveolar membrane had become flattened and pulled into the fracture plane. The pits do not appear to be involved in pinocytosis; even after prolonged soaking of the connective in peroxidase solutions, the only layer of reaction product was that bound to the pits which communicated with the extracellular space. The results signify that the pits can be unfolded during stretch and thus increase the extensibility of the sarcolemma. Because of its structure, the pit membrane should be distinguishable from isolated fragments of sarcolemma and so lends itself to biochemical assessment of a possible role in stimulus - contraction coupling.